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The true intra-individual change model is generalized by defining individual method effects. This allows the analysis of non-congeneric test–retest variables assumed to measure a common, possibly (temporally) transient, attribute. Temporal change in the attribute between different times of measurement is modelled by the true-change variable. Individual causal method effects, due to heterogeneity of the measurement methods, account for the imperfect correlation of the true-score variables at each time of measurement. The reliability of the composite scores, at each time of measurement, and the reliability of the difference composite score may be estimated with appropriate coefficients derived from the model. Measurements of daily life tension in adult females serve to illustrate how the model can be used empirically.